1. Field of the Invention
The present invention relates to a method for spatially resolved measurement of the field distribution of high frequency pulses emitted from a high frequency antenna of a magnetic resonance scanning. In addition the invention relates to a magnetic resonance apparatus with a high frequency antenna and components for spatially resolved measurement of the field distribution of emitted high frequency pulses.
2. Description of the Prior Art
Magnetic resonance imaging, also called magnetic resonance tomography, is a technique that is now widespread for acquiring images of the body interior of a living object to be examined. In order to acquire an image with this method, the body or the body part to be examined must first be exposed to a homogenous static basic magnetic field (usually characterized as a B0 field), which is generated by a basic field magnet of the magnetic resonance measuring instrument (scanner). During the data acquisition for the magnetic resonance images, rapidly switched gradient fields for local coding are superimposed on this basic magnetic field, these fields being generated by gradient coils. Moreover, with high frequency antennae, high frequency pulses of a defined field strength are irradiated in the object to be examined. The magnetic flux density of these high frequency pulses is usually termed as B1. The pulse-shaped high frequency field is therefore generally also called the B1 field for short. By means of these high frequency pulses the nuclear spins of the atoms in the object to be examined are excited in such a way that they are deflected by a so-called “excitation flip angle” (in the following also referred to as “flip angle”) from their state of equilibrium parallel to the base magnetic field B0. The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals generated as a result are picked up by high frequency receiving antennae. The receiving antennae can either be the same antennae with which the high frequency pulses are irradiated, or separate receiving antennae can be used. The magnetic resonance images of the object to be examined are finally created on the basis of the received magnetic resonance signals. Each pixel in the magnetic resonance image is assigned to a small body volume, a so-called “voxel”, and every brightness or intensity value of the pixels is linked with the signal amplitude of the magnetic resonance signal received from this voxel. The relationship between a resonant irradiated high frequency pulse with the field strength B1 and the flip angle a achieved with it is given by the equation
                    α        =                              ∫                          t              =              0                        τ                    ⁢                      γ            ·                                          B                1                            ⁡                              (                t                )                                      ·                          ⅆ              t                                                          (        1        )            wherein γ is the gyromagnetic relationship, which for most magnetic resonance examinations is a fixed matter constant, and τ is the exposure time of the high frequency pulse. The flip angle achieved by means of an emitted high frequency pulse and hence the strength of the magnetic resonance signal consequently not only depends on duration of the pulse, but also depends on the strength of the irradiated B1 field. Spatial fluctuations in the field strength of the exciting B1 field therefore result in undesired variations in the received magnetic resonance signal, which can falsify the results of the reading.
Inconveniently, however the high frequency pulses, particularly in the case of high magnetic field strengths—which due to the required magnetic base field B0 are necessarily present in magnetic resonance tomography—show an inhomogeneous penetration behavior in conductive and dielectric media such as tissue, for example. As a result of this, the B1 field can vary greatly within the measuring volume. To be able to take these variations of the B1 field into consideration in the measurement for example in an adjustment of the B1 field or in an evaluation of the received magnetic resonance signals, it would be very advantageous if this effect could be quantified.
Currently, for calibration of the high frequency pulse voltage and the average B1 amplitude or the average target flip angle which is to be achieved by the HF pulse, conventionally a so-called “transmitter adjustment” is automatically performed within an adjustment sequence of the magnetic resonance measuring apparatus. For this purpose, a first high frequency excitation pulse is first emitted within a so-called double echo high frequency pulse sequence, which deflects the nuclear spins by a flip angle α1. Then after a specified time a second pulse takes place, a so-called “refocusing pulse”, which results in a further flipping by 2·α1.
After measurement of a first echo (so-called spin echo) an additional α1 refocusing pulse is emitted and a second echo (the so-called stimulated echo) is measured. The following applies for the amplitude of the measured spin echo signal ASE and the measured stimulated echo signal ASTE as a function of the flip angle α1ASE=eiφ sin3(α1)  (2a)ASTE=eiφ sin3(α1)cos(α1)  (2b)This dependency is graphically represented in FIG. 2. The flip angle a1 achieved with such a pulse sequence can hence be determined by the condition
                              cos          ⁢                                          ⁢                      α            1                          =                              A            STE                                A            SE                                              (        3        )            from the relationship of the amplitude of the two echo signals. This flip angle a1 can be converted into the irradiated B1 field with the help of equation (1).
However, in this classic transmitter adjustment it must be noted that the magnetic resonance signal stems from the whole excitation volume and not just the relevant part near the isocenter of the magnet, i.e. of the central area in the MR device. Due to the finite bandwidth of the high frequency pulse, moreover, a flip angle distribution over the excited volume must be assumed. For this reason in practice, as a rule a weak, constant slice selection gradient is switched. As a result the high frequency pulses are slice selective and instead of the entire excitation volume, only a central transversal layer with a thickness of for example 10 cm is excited. This constant slice gradient is also created during the data collection. As a result the possibility of spatial resolution exists along the slice standards, i.e. along the thus defined z-axis parallel to the B0 field. In practice the collected echo signals are Fourier transformed and only the central column of the transformed signals is evaluated. This central column contains the signal contributions of an approximately 1 mm thick slice section, within which a restricted pulse bandwidth does not play a significant role.
In addition, due to the fact that the excited nuclear spins gradually tilt back (relax) parallel to the basic magnetic field, in particular the so-called T1 relaxation (longitudinal relaxation) influences the two echoes in variable form. As a result the results are falsified. Through the T1 relaxation, which is tissue-dependent, the measurable amplitude
  ⅇ            -      t              T      1      of the stimulated echo ASTE is lower by the factor of e than without a relaxation effect. In reality this correctly results in the following for the relationship of the amplitudes
                                          A            STE                                A            SE                          =                              cos            ⁡                          (              α              )                                ⁢                      ⅇ                                          -                t                                            T                1                                                                        (        4        )            wherein t is the duration between the 2nd and the 3rd high frequency pulse. If one evaluated the flip angle unchanged according to equation (3), values would be measured that are systematically too large for flip angle <90° and values that are systematically too low for flip angle >90°. In practice therefore the amplitude of the stimulated echo under the assumption of a “mean” T1 value would be corrected up by approximately 5%. Moreover, the method is applied iteratively until a flip angle of α that is approximately equal to 90° is found. That is, a first measured mean flip angle α1 is used as an approximate value for the actual flip angle, which due to the influence of the T1 relaxation is not precisely measurable. A high frequency pulse voltage is then estimated from the first measured flip angle α1, said high frequency pulse voltage being necessary to obtain a specified flip angle α2=90°. The whole measurement is then repeated with the target flip angle α2. This procedure is iterated until finally a flip angle a of approximately 90° is achieved. The influence of the T1 relaxation is then minimal and precisely the high frequency pulse voltage U is determined, which is necessary to generate a flip angle of 90° with the used high frequency pulse shape and duration.
Since the transmitter adjustment is performed only for the central, average layer, this procedure however can only guarantee that a correct average target flip angle is set in a measurement. Space-dependent variations of the B1 field within the measuring volume of interest cannot be taken into consideration. The adjustment method hence still is acceptable with the currently most widespread field strengths of the base magnet field of up to 1.5 tesla, since here the B1 field is relatively homogeneous. In the case of field strengths of 3 tesla, however, as used in newer intense field devices, a more precisely spatially resolved specification of the B1 field is needed.